1. Technical Field
This invention relates to electrical isolation devices and particularly to an improved optocoupler device having an output gain dependent upon loop length.
2. Background Art
It has become known that it is possible to design an isolation device for passing signals from an input to an output using optocoupling at the barrier between the input and output terminals. Such a device is the subject of U.S. Pat. No. 4,056,719 issued to S. G. Waaben on Nov. 1, 1977. The Waaben patent provides a solution to the problems of linearity of the output signal with respect to the input signal. Under the Waaben teaching a pair of light detecting diodes and a LED (light emitting diode) are used in a feedback arrangement such that the light output characteristics of the photodetector in the output circuit linearly track input signal. Thus, as taught by Waaben, an optocoupler can be built that corrects for light output differences between the LED and the photodiodes as well as corrects for temperature coefficient differences between devices.
In our concurrently filed copending patent application, Ser. No. 893,795, now abandoned, we have disclosed an arrangement for gain controlling the optocoupled circuit under control of a DC reference voltage in the output circuit. This copending application is hereby incorporated into and made a part hereof by reference thereto.
As discussed in our aforementioned patent application, one usage of the optocoupled device is as a barrier between the input and output sides of a telephone line for isolation purposes. The use of our device in such a manner solves many of the problems inherent with such isolation devices. However, a problem that exists in all transmission lines is that as the loop length increases the quality of the transmission deteriorates. This deterioration is related to the resistance of the line itself and is manifested in a drop in the DC voltage level from one end of the line to the other. In addition, as the length of the transmission line increases there is a fall off in signal gain due to the frequency of the transmission.
Accordingly, a need exists in the art for a circuit capable of automatically correcting for changes in loop length. A further need exists for a circuit capable of correcting both the AC and DC transmission signals passing over variable length lines.